CF patients with impaired glucose tolerance due to defective insulin secretion are not routinely treated with exogenous insulin since fasting glucose and hemoglobin Alc levels are normal. This practice ignores the potentially detrimental effects of insulin deficiency on protein metabolism. We hypothesize that chronic treatment with either insulin or an oral hypoglycemic agent will lead to improved protein anabolism in insulin-deficient, non-diabetic CF adults. A multicenter, prospective, placebo-controlled, double-blinded intervention trail will be undertaken to treat glucose intolerant adult CF patients for six months with one of three agents: insulin, glipizide, troglitazone or placebo. We will evaluate the effects of these agents on protein synthesis and breakdown, pulmonary function, and body composition.